Currently isn't... but Apple is moving toward AppStore on OS/X...
It will just make Linux and open source more popular again and Apple will slowly start loosing the developers and power users, again. Others will eventually follow.
Combined with already risky move to ARM, they have to be crazy to go that route.
I think digital signatures are purely a security measure, and the dev account requirements are a way to establish some kind of chain of trust to the developer, to prevent bad actors from signing up on the spot and pushing out tons of malware unimpeded.
My guess is that Apple will make it step by step more difficult to install non-store apps so that people get used to it and only then cut it off.
It would be nice for Apple to not do this, but if they can keep their ARM chips highly competitive it would be insanely lucrative to do so. Corporations are for-profit entities first and foremost.
It wouldn't be lucrative, because all the pro users (basically the mac userbase) would just not buy one. What's the point in getting one over an iPad?
Better keyboard, larger screen, more connectors. And despite a lot of devs using macs, most of the userbase aren't devs, but are instead designers, artists, or just average people like college students etc. Most of those don't need third party stores, or execute any type of software outside of an apple approved list. Give them their specialist DAW, Adobe suite, etc. and they are content.
If apple wanted to shut down alternative app stores macports and homebrew would already be dead.